A few years after graduating from the University of Mississippi’s College of Veterinary Medicine in 2000, Cherie Buisson, DVM, became acquainted with feline heartworm disease.

Buisson was working at a small-animal practice when a couple brought in their one-year-old, neutered, indoors-only cat. He was dead on arrival. Buisson conducted a necropsy and found four to five worms in his heart.<

“I was shocked,” says Buisson, now the director of veterinary services for the Tampa Bay SPCA in Florida. “He was perfectly healthy up to that point. I’m certain that’s what killed him. [We’d known about feline heartworm], but it wasn’t until this instance that it really hit home with me. I realized that we needed to push for prevention.”

Heartworm disease affects cats differently from dogs; therefore, many believe that cats aren’t prone to it. In cats, the name “heartworm disease” is a misnomer, as the disease mostly affects the lungs and not just the heart. But the condition is easily avoided with a year-round broad-spectrum preventive.

Michael Paul, DVM, executive director of the Companion Animal Parasite Council (CAPC), says that awareness has increased as the disease spreads across the country. “Where it was centered in the southeast of the country, it’s now all over the 50 states,” he says. “Climates have changed. Even in places where mosquitoes haven’t been a problem, there are now more places where they’re capable of transmitting heartworm.”

Infection starts the same way as it does in dogs—a bite from a mosquito carrying microscopic heartworm larvae. Once the larvae develop into immature worms, they enter blood vessels.

In cats, the immature worms are carried to the arteries in the lungs, where they induce an inflammatory reaction that can cause coughing or difficulty breathing. Most worms die at this stage, often resulting in more respiratory problems. Worms that actually make it to the adult stage may live undetected for a couple of years. In most cases, the death of the adult worms causes inflammation severe enough to kill cats.

“Cats are affected by the larvae more than by the adult worms,” says Jane Brunt, DVM, of the Cat Hospital at Towson (Baltimore, Md.). Dr. Brunt is past president of the American Association of Feline Practitioners and a spokesperson for the Know Heartworms Campaign. “They get heartworm-associated respiratory disease (HARD). Essentially it’s a lung disease in cats, not a heart disease.”

Signs are often mistaken for feline asthma or allergic bronchitis—or even hairballs. “A lot of times with coughing [in a cat], people think it’s just a hairball,” says Brunt. “And that’s the hallmark for asthma and HARD. People will shrug it off. A hairball is not something that should be happening in cats, anyway. They should be able to pass it through without any problems.”

Signs of feline heartworm:

Coughing

Difficulty breathing

Fainting

Lethargy

Rapid heart rate

Vomiting

Anorexia

Diarrhea

Blindness

Convulsions

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Tests have limitations, so a negative result doesn’t necessarily rule out an infection. And along with unclear test results, a lot of cats do not exhibit the telltale symptoms of heartworm disease until it is too late. In fact, among the list of signs is sudden death.

Some cats can recover with heartworm preventives and anti-inflammatory drugs, but such treatments are often risky. “We try to reduce their symptoms to the point where they don’t die from them, but we’re not always successful,” says Deborah Edwards, DVM, ABVP (Feline Practice), who owns feline-only All Cats Hospital based in Largo, Fla. “A much better option is to use preventives. After all, one of the more common symptoms for feline heartworm disease is sudden death, and we can’t do much more care for that.”

Cat owners should get their pets on a heartworm preventive year-round—and that goes for indoors-only kitties, too. “Indoor cats should absolutely be put on a preventive unless you can guarantee that you will never get any mosquitoes in your home,” Edwards says. “And who hasn’t woken up with a mosquito bite?”